The officials suspect that heating companies were dying the water red in their boilers before discharging it to warn people not to use it, the report said.

Local environmental officials were testing samples of the water to find if the discharge was detrimental to the water quality of the river, it said.

The discharge comes as northern China braces for the winter and public heating systems prepare to go on-line.

Local Lanzhou officials were not immediately available to comment on the spill.

The spill is the latest in a seemingly endless series of reports about China's worsening water situation.

More than 70 percent of rivers and lakes are polluted, while underground water supplies in 90 percent of Chinese cities are contaminated, previous reports have said.

A recent report by the United Nation's Environmental Program declared the estuaries of the Yellow and Yangtze rivers "dead zones" due to the amounts of pollutants they discharge into the sea.

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